A rain barrel is a container designed to capture and store rainwater runoff from a rooftop, typically collected via a downspout. Harvesting this water provides a readily available source for non-potable uses like garden irrigation and car washing, reducing reliance on municipal water systems. Determining the appropriate size for a rain barrel is a calculation based on two specific local variables: available roof area and average precipitation.
Key Data Points for Sizing
The first step in sizing a rain barrel is determining the square footage of the roof area that will funnel water to the downspout, known as the catchment area. This measurement is crucial because only the portion of the roof connected to the barrel will contribute to the water supply. To find this, measure the length and width of the roof section that drains into the specific gutter and downspout you plan to use. It is important to note that the slope or pitch of the roof does not affect the calculation, as rain falls evenly across the horizontal footprint of the collection area.
The second necessary data point is the average precipitation for your geographic location, typically measured in inches. This figure should ideally reflect the average rainfall per storm event or the monthly average during the watering season. Reliable sources for this data include the National Oceanic and Atmospheric Administration (NOAA) or local meteorological offices, which often provide long-term precipitation statistics. Using a typical rainfall amount for a single event allows you to calculate how quickly a barrel will fill, which is more practical than using an annual total.
Calculating Potential Water Yield
Once the key variables are known, a simple formula translates the roof area and rainfall into a potential volume of water in gallons. The principle behind this calculation relies on the fact that one square foot of surface area receiving one inch of rain yields a specific volume. Specifically, one inch of rain falling on a 1,000-square-foot roof can yield approximately 623 gallons of water.
The formula for calculating potential water yield is: (Roof Footprint in sq. ft.) x (Rainfall in inches) x 0.623 = Total Gallons Captured. The factor 0.623 serves as the conversion constant to move from cubic feet of water (area x height) to gallons. For example, a 500 square feet of roof area receiving a single one-inch rain event would produce about 311.5 gallons of water.
It is important to understand that this calculation represents the theoretical maximum volume collected. In reality, factors like wind, splash-out, and the efficiency of the gutter system mean not every drop is captured. To account for these minor losses, some experts apply a runoff coefficient, typically between 0.75 and 0.95, which suggests a small percentage of water is lost before reaching the barrel.
Matching Yield to Water Needs
The final step in selecting a rain barrel size involves balancing the calculated potential yield with the actual water usage needs. A rain barrel should be sized not just to capture water but to hold what is needed before the next expected rainfall. The intended use of the water dictates whether a smaller or larger capacity is appropriate for the system.
For users planning on daily or frequent use, such as watering a small vegetable garden, a smaller barrel may be appropriate. If the barrel is regularly emptied, it has a capacity to recharge quickly with the next storm event, making a standard 50- to 80-gallon barrel sufficient. This approach focuses on rapid turnover rather than long-term storage.
Conversely, if the intent is to store water for use during extended dry periods, a much larger storage capacity is necessary. In this scenario, the volume collected from a single storm must cover the longest expected period without rain. This often requires connecting multiple barrels in a series, creating a cistern system that can hold several hundred gallons of water. Common commercially available rain barrels range from 50 gallons up to 100 gallons or more.
When choosing a final size, select a barrel slightly larger than the calculated volume of water needed for a typical use cycle. This buffer capacity prevents the system from overflowing during moderate rain events and provides a reserve. By considering both the potential water supply from the roof and the specific demand for the water, the chosen barrel size will be optimized.